A variety of products contain electrical circuitry for carrying signals and/or power to perform functions such as communication, display, heating, computation, etc. Electrical circuitry can be wired by hand, but is typically embodied in what is known in the art as a printed circuit board, which is installed in the product. Conventional printed circuit boards were made from a rigid, non-conductive substrate upon which conductive pathways (e.g., metal) have been formed.
A variety of processes have been used for forming the conductive pathways on the non-conductive substrate. For example, a metal film such as copper can be applied to a rigid, semi-rigid or flexible, non-conductive substrate such as fiberglass, epoxy, and/or polyamide. In a common process, a sheet of the conductive metal is laminated to the non-conductive substrate and a photoresist is then coated on the metal sheet. The resulting printed circuit board is then exposed to a pattern of light employing a light mask to reproduce the metal pathway pattern desired. This exposure is followed by photoresist development and then metal etching in the area unprotected by the photoresist, in order to produce the desired circuit pattern. In the alternative, an etch resist can be directly printed such as by silk screen, gravure, or the like, on the metal laminate sheet followed by curing and then metal etching. Of course, this multi-step process is slow, time-consuming, relatively expensive, and contaminate to the environment.
Another process presently available uses metals or metal salts, which are dispersed as particles in a solution, usually in a polymeric binder, and the particles function as seed sites for subsequent plating with a metal. The polymeric composition containing the metal or metal salt is applied to a substrate in the pattern desired. The composition is then heat-cured in order to drive off solvent and to cross-link the polymer. The substrate is then submerged in a metal bath or solution where metal pathways grow between the seed sites. This is a multi-step process that is not only slow but also expensive. Typical examples of these processes are disclosed, for example, in U.S. Pat. Nos. 3,900,320; 3,775,176; and 3,600,330.
Electrically conductive metal pathways can also be formed by a process which includes coating a substrate with a composition containing a reducible metal complex. For example, the substrate can be coated with a sorbitol copper formate solution containing a photo-activated reducing agent. Upon exposure to ultraviolet radiation, unmasked areas are reduced to copper metal and are suitable for plating nucleation sites. Non-exposed areas are washed clean and all copper formate is removed before plating can be carried out. Again, such processes are time consuming and expensive. Examples of this technology may be found in U.S. Pat. Nos. 4,268,536; 4,181,750; 4,133,908; 4,192,764; 4,167,601; and 3,925,578.
Printed circuit boards can also be produced by silk-screen processes in which a silk screen is placed on top of a rigid substrate and ink is pushed through open areas of the silk screen onto the substrate. This is an indirect printing process because the silk screen stencil must first be placed over the substrate, a high viscosity ink is then pushed through the screen onto the substrate, and then the silk screen stencil must be removed. Several problems are associated with this process. First, the inks must be fixed so as not to flow through the screen except where pushed, yet they must be applied with sufficient quantity and thickness of ink to flow together after being applied to make a uniformly conductive surface. Next, the speed of production is very slow with only a small quantity of printed circuit boards being produced with given period of time. Finally, the precision of the circuits is quite low since pulling the stencil away from the substrate causes dispersion at the edges. Silk screen processing cannot produce thin or narrow lines because of the high viscosity ink. Also, silk screen inks are quite expensive and difficult to process. Silk screen processes cannot be used to produce multi-layer printed circuit boards, and silk screen processes can only be used with rigid substrates since a firm backing is required to push the ink through the silk screen stencil and to remove the stencil. Even if these difficulties can be overcome, silk-screening is difficult or impossible to automate fully for high speed printing.
In yet another known process, catalytic inks are applied to a substrate and are subsequently activated for plating by electroless surface treatment with noble and/or non-noble metals or salts. The coated substrates are heat-cured and require elevated temperatures and long cure times. These multi-step processes are quite expensive and time consuming. Typical examples of these processes are shown in U.S. Pat. Nos. 4,089,993; 4,073,981; 4,100,038; and 4,006,047.
Printed circuit boards may also be produced by an additive technique wherein a metal-loaded resinous ink is first printed on the board, the circuit next being covered with a conductive metal powder while the ink is still wet. The powder is then pressed into the ink and the circuit cured. Next, a solder stratum is alloyed with the powder as by a solder paste printed over the circuit and the board heated to cause the solder to alloy with the ink an powder substrates. A solder resist may then be applied selectively over the circuit and multiple layers of circuits may be built up on the board. The conductive ink is an epoxy resin loaded with a metallic powder, preferably copper, with a catalyst added to the ink. The solder paste is a lead-tin alloy containing antimony suspended in a binder and a flux. The apparatus employed to carry out the procedure includes silk screens and a roller arrangement for pressing the metallic powder into the ink. An example of this process is disclosed in U.S. Pat. No. 4,327,124.
Furthermore, U.S. Pat. Nos. 6,010,771 and 5,763,058 disclose a method of making printed circuit boards formed by a conductive liquid printed directly onto one side of the substrate. The electrical component is then capable of performing its electrical circuit functions, as printed, and without the necessity for post-printing processes such as metal etching, catalytic ink activation, or electroless deposition.
In summary, known processes for producing electrical circuitry such as silk-screen, catalytic ink, chemical etching, electroless bath, etc. are expensive, time consuming, substrate restrictive and sometimes environmentally harmful. What is needed is an efficient, environmentally friendly method for producing electrical component-bearing substrates, and especially printed circuit boards that allows a wide variety of substrates to be used, including previously unavailable inexpensive substrates, which can be performed at lower electrical currents and temperatures than the prior art.